PDFs were designed by Adobe for secure electronic exchange of documents that were made to be printed. They really don't even work well on computer monitors without scrolling. If the document was designed to be printed on a 8.5 inch by 11 inch piece of paper at 300dpi, then you are talking about 2550 dots by 3300 dots. In comparison the Sony Readers have 200dpi screens that are 3 inches by 4 inches which translates into 600 dots by 800 dots. We can start calling dots pixels and the comparison is that viewing a 2550x3300 pixel document on a 600x800 pixel e-Ink screen isn't going to be a great experience. Not only is the size difference astronomical, the width to length ratio is also different. Even the expensive iPad with its 9.7 inch screen will not display an 8.5x11 inch PDF very well without scrolling.

As mentioned, this applies to all small screen devices whether a Sony or another brand. There is no way you are going to satisfactorily view an 8.4 megapixel document on a 0.5 megapixel screen. It is pretty much like trying to view a large digital photo you took with your camera using the tiny display on the back of the camera. You will get the gist of the photo, but detail won't come through unless you scroll and zoom.